owing to the excellent optoelectric characteristics of a-Si films, they are used in solar batteries, photosensors for electrophotography, thin-film transistors, optical sensors, etc. As one of fabrication processes of such a-Si films, may be mentioned the so-called chemical vapor deposition process (hereinafter abbreviated to "CVD") in which a gas such as silane is thermally decomposed to deposit amorphous silicon on a substrate. The CVD process has such great merits, compared with the plasma deposition technique, sputtering deposition technique, ion-plating process and the like, that the CVD process may be readily practiced without using complex, expensive and large facilities.
The CVD process is however accompanied by such drawbacks that the materials of substrates, on which a-Si is to be deposited, are limited to expensive metals, silica glass, sapphire or the like because the CVD process requires extremely high temperatures of 600.degree. C. and higher. It is the so-called photochemical vapor deposition process that was proposed with a view toward solving such drawbacks and enabling to practice the CVD process at lower temperatures and has recently been attracting attention. Among such photo CVD processes, the leading one is currently the mercury sensitizing process. This process makes use of the sensitizing effect of mercury (Hg). When Hg vapor is also allowed to exist in a thermal decomposition reaction system and a light is radiated to Hg vapor to excite same ##STR1## transfer of energy is induced to occur between the excited species and the reaction species so that the thermal decomposition reaction may be allowed to proceed. This action of Hg is a sort of catalytic effect and can increase the growth rate of the film by several to several thousands times. Accordingly, an a-Si film can be formed at a relatively low temperature (for example, 400.degree. C. or so) for example in a thermal decomposition reaction of SiH.sub.4.
The mercury sensitizing process involves an inherent problem from the viewpoints of pollution and safety aspects that it requires to use mercury, which is a toxic substance difficult to handle, and moreover to employ it in a vapor form, and it is accompanied by a drawback that it is indispensable to apply a surface treatment and post treatment to resulting a-Si although such treatments are difficult to conduct--(1). Besides, the mercury sensitizing process is accompanied by a serious shortcoming from the practical view point that, since Hg vapor is caused to exist all over the interior of the reactor cylinder, a-Si is allowed to deposit on a window or the like through which a light is introduced into the reactor cylinder and hence gradually deposit there, interferes the introduction of the light and lowers the growth rate rapidly in the course of each decomposition operation, resulting in unavoidable frequent interruption of the operation--(2). In addition, there is a possibility that Hg may be taken in the resulting a-Si film--(3).